Wet scrubber

ABSTRACT

A wet scrubber for absorbing oxides of sulphur from contaminated gas comprises an upright housing having an inlet for contaminated gas adjacent at the lower end and an outlet for clean gas adjacent the upper end, a plurality of vertically stacked, successive intense wet scrubbing zones in said housing between said inlet and outlet, each of said zones comprising a horizontal array of elongated, parallel, spaced apart rod like elements which substantially reduce the total flow cross section available to the gas moving therebetween to a value less that of the cross section above and below the array, upper spray means adjacent an upper of said scrubbing zones for distributing into said gas finely divided droplets of scrubbing liquor, which liquor comprises a limestone slurry, said liquor flowing in a downward direction and cascading countercurrent to the upward gas flow through each successively lower scrubbing zones, a high velocity particulate collector stage in said housing between said inlet and a lower of said wet scrubbing zones comprises baffle means across said housing having a flow restricting opening therein and lower spray means for directing scrubbing liquid into the high velocity gas flowing through said opening in a concurrent direction therewith.

United States Patent 1 1 15% SLURRY HOLD 1 N G TAN K 4; PULVERIZER) DAYSTORAGE LIMES ONE STORAGE Ekma'n 1 Mar. 5, 1974 1 1 WET SCRUBBER I OTHERPUBLICATIONS 75 Inventor: Frank 0. Ekman, Barrington, n1. y 2 Remq alyst m Filtration & p

t' ,V.9,N.4, 484. [73] Assignee: Environeermg lnc., Skokre, 111. O1 0July/August 9 page [22] Filed; F b, 22, 1973 1 Primary Examiner-DennisTalbert 52 us. 01 23/283, 23/284, 55/22, A Wet Scrubber for absorbingOxides of Sulphur from 55 73 9 5 22 55/223, 55/229 contaminated gascomprises an upright housing having 55 231 55 2 0 5 24 55/257 241/D1G'2, an inlet for contaminated gas adjacent at the lower 261/111 261/112261/115, 423/242 end and an outlet for clean gas adjacent the upper 51int. Cl 801d 47/06, B01d'47/12 end, a plurality of vertically stacked,Successive 5 1 i m f Search 423 42 4 2155 511, 544, tense wet scrubbingzones in said housing between 423 554 555 551 545;. 5 3 2 said inlet andoutlet, each of said zones comprising a 223, 228, 230, 231, 233, 240,241, 242, 257, horizontal array of elongated, parallel, spaced apart 44022 34 35 39 9Q, 91 93 9 261/81, rod like elements which substantiallyreduce the total 83, 89 90, 108 109, l 10 H1 H2, 1 l3, 1 14 flow crosssection available to the gas moving therebe- R 1 14 VT 115'; 241/ 2; 3233 284 tween to a value less that of the cross section above and belowthe array, upper spray means adjacent an 5 Referen'ces Cited upper ofsaid scrubbing zones for distributing into said UNITED STATES PATENTSgas finely divided droplets of scrubbing liquor, which liquor comprisesa limestone slurry, said liquor flowing 2316491 4/1943 T h' 2611122523441 9/1950 l 55/223 in a downward direction and cascadingcountercurrent 7'895'S66 7/1959 g gr s 4 X to the upward gas flowthrough each successively 57 5 2/1952 Kotzcbuuc VT lower scrubbingzones, a high velocity particulate col- 3I353j799 11/1967 Lions ct a1...261/3 lector stage in said housing between said inlet and a 3 03 1/1970Ekman I h I 261/31 lower of said wet scrubbing zones'comprises baffle3,544.087 12/1970 Mcllvaine.... 55/233 X means across said housinghaving a flow restricting 3,687,613 8/1972 Rickard 423/242 openingtherein and lower spray means for directing scrubbing into the velocitygas flowing through said opening in a concurrent direction there-885,548 5/1943 France. 261/ll4 VT with 7 Claims, 7 Drawing Figures @0 61 I 7 lo ggg 4/, 14 28 -1-8 4342,

IN 4 456 10 E 2 102 66% SLURRY HOLDING TANK A I STORAGE EB PATENTED51974 SHEET 1 OF 2 ooo 000000 :52 O oooPooo oloj 00 0000 o o 001::.

1 52 0o ookoooofoo o o EFFECT oF GAS VELOCITY AND WATER RATE 1ABSORPTION EFFICIENCY vs.PREssuRE LOSS EIGHT 3' J'RoD STAGES-4l% OPENAREA 'CL/6= 2o 0 L/G= 2o *IGFPS, L/G 98 OL/G OL/G =40 Ie" SPACING AL/GALIG=60 BETWEEN sTAsEs 1 [L 9O A @L/G-BO, IO FPS m 5 I --50%OPENVRGRIDSg ohgy a l6 FPS\, 2 I OL/(MOI lo FPS g AL/G=60, IO FPS y I:II./e=Ioo, IoFPS L/C= 2o 0 -zs%oPENvRsIIII s 0L/G=40, l4 FPS so AL/G=60, I4 FPS 8I./e=ao, l4 FPS 4o 3% nL e=Ioo,I4I-Ps cu oIJs= 40, I6 FPS 8-3 AL/G=60,l6 FPS l I I l I I l I I E i 5 s 7 a I 2 a 4 s s 7 s ABsoRBE PRE suRELOSS PRESSURE LOSS 0F ABsoRBER IN INCHES OF WATER INCHES oFwATER 99EFFECT OF NUMBER ROD LAYERS(STAGES) (46%|PPEN AREA, sQuARE R005 l6SPACING BETWEEN LAYERS) 98 F 2 m O 0! Lu 9sa 5 a ROD I AYERs 6ROD LAYERS)OGAS VEL-IO FPS g GASVEL-IO FPS 92 5 ROD LAYERS 5 GAS-VEL-IO FPS II aLAYERS oL/C=4o, IO FPS 2 AI 5=60, 9. .L/G =80, N E lL/e=I0o, Ir 'kL/G=4OI2 FPS o LAYERk 2 o L/G=40,IO|FPS A L/G =60,

oL/s --4 LAYERS o L/G =40,|O FPS A L/G =60,

o I I I I O I I 2 a 4 s s 7 PREssuRE LOSS ACRoss ABsoRBER, we

FIG. 5

EFFECT OF OPEN AREA RATI O ABSORPTION EFFICIENCY VS PRESSURE LOSS EIGHTV4" ROD STAGES PATENTEDHAR SIIII SHEEI 2 F 2 'IIIIIII. 4

a will. wanna!!! 54 9s I I17; rf94 H1; 1 I 96 9O 66% SLURRY '42 HOLDINGTANK km I36-- I 126k PuLvERIzER D UNLOAD STORAGE STORAGE no I" LIMESTONESTORAGE FIG. 6 Fl 6.?

c NIPARIsoN OF ABSORBERS g9, ABSORPTION vs AT IOFPS SPACE VELOCITY INABSORBER AB O BER PRESSURE LOSS ND 20 PS. PUMP PRESSURE AT l0 FPS SPACEVELOCITY 98 'HP =THEORETICAL Hp 98 or PUMPING AIR AND WATER E 96 :p::pair +uHpwaz-br ll 1 z fi' ia l'fi A-ElGHT A ROD vRs s4 %,,522240 OF4|%OPEN 94 o HJJ E B40 B.4O.ElGHT0F/ "RQD 92 ABSORBER C.E|GHT46% OPEN 'E I,wIRE GRIDS a: 9'9 E.TWOSTAGE OFTCA g a ABSORBER WITH I2" 8O... ,0 3 llso 3 A PACKINGEACH STAGE 8 A6 VRS,40%OPEN g EoNE MARBLE BED w L/G 2o 40so so E NOWATERDEFLECTOREI o A 6o A C E F 0 II II D A m 0 O 4o 0 e e O oE 8 46%0PEN wIRE GRIDS 060 A A A A L/G204060 so 9 Q Q a e A 00 I I 2 3 oI l l l g 9 3 4 s 6 HP PER MCFM ABSORBER PRESSURE LO IN INCHES OF WATERWET SCRUBBER The present invention is directed towards a new andimproved wet scrubber system for contaminated gas and more particularlyis directed towards a wet scrubsorbing oxides of sulphur, a scrubbingliquid comprising a limestone slurry is intimately contacted with thecontaminated gas as the gas moves upwardly in an absorber tower whilethe scrubbing liquidflows downwardly overloose ceramic packing material.These types of apparatus for absorbing sulphur oxides in gas areunsatisfactory because of the fact that thetower elementsor packing.material quickly plugs up with calcium sulphate and other scale formingmaterials.

. Cleaning of the elements and packing material at frequent intervals iscostly and time-consuming. As the scale collects on the packing materialthe pressure drop becomes excessive and the amount of power required perunit volume of gas being treated is so high that the scrubbers areunable to operate on an economical basis. In addition, it isuneconomical to clean the packing material andcomplete replacement ofthe elementsis often necessary. U

' lt is therefore an objectof the present invention to provide a new andimproved wet scrubber system capabl e of removing oxides of sulphur fromindustrial gases and the like in an efficient and economic manner.

Another object of the invention is to provide a new and improved wetscrubber which is highly efficient in the removal of particulatecontaminants as well as 'gaseous contaminants such as oxides of sulphurfrom industrial gases and the like.

Still another object of the invention is to provide a new and improvedwet scrubber of the character described which is highly efficient inremoval of contaminant particles and gaseous contaminants fromindustrial gases which is operable at extremely high efficiency in termsof power requirements per unit of gas being treated and in terms ofquantity flow rate of liq uid required per unit of gas being treated.

Another object of the present invention is to provide a new and improvedwet scrubber of the character described which is highly resistant toplugging or clogging up even though limestone slurry is used as ascrubbing liquor.

Another object of the present invention is to provide .a new andimproved scrubber of the character described employing plurality ofvertically stacked, wet scrubbing stages spaced apart sufficiently toprovide successive gas absorption zones of high velocity gas flowalternated with deceleration zones of low velocity wherein little wearor abrasive action on the scrubber occurs and wherein little if anytendency towards plugging of the system with scale or deposits ofprecipatates occurs.

Another object of the present invention is to provide a new and improvedwet scrubber of the character described wherein scrubbing liquor iscascaded downwardly through a series of successive scrubbing zones orstages employing aplurality of rod elements in horizontal arrays forproviding impingement surfaces for disbursing and deflecting thedownwardly cascading scrubbing liquid moving toward the lower end of thescrubber counter tothe direction of upward gas flow.

Another object of the present invention is to provide a new and improvedwet scrubber of the character described which lends itself to rapidcleaning and easy and rapid replacement of the rod-like elements in eachof the several successive scrubbing zones.

Another object of the present invention is to provide a new and improvedwet scrubber of the character described which scrubber isicapable ofremoving large quantities of oxides of sulphur with a relatively lowpressure drop through the scrubber being required.

Another object of the invention is to provide a new and improved wetscrubber of the character described which scrubber is well able towithstand high temperature gases and which scrubber isnot highlysensitive in terms of performance to increased heat and temperatures ofthe gas being treated.

These and other objects and advantages of the present invention areaccomplished in an illustrated embodiment comprising a wet scrubber forabsorbing ox-' ides of sulphur and particulates from contaminatedindustrial gases and the likeincluding an upright housing having aninlet for contaminated gas adjacent to the lower end and an outlet forclean gas adjacent the upper end. A plurality of vertically stacked,intense wet scrubbing stages or zones is provided in the housing betweenthe inlet and outlet and each zone or stage includes a horizontal arrayof elongated, parallel spaced apart rod-like elements, which elementssubstantially reduce the total cross sectional flow area in the housingat the level of the array. Upper spray means adjacent an upper of saidscrubbing zones is provided for distributing finely divided droplets ofscrubbing'liquor into the gas flow; The liquor may comprise water and/ora mixture of water and limestone in a slurry which cascades downwardlyonto the elements in each successive lower scrubbing zone while the gason the other hand flows upwardly in a counter direction through thescrubber housing. I

Adjacent the lower end of the housing there is'provided a high velocity,particulate collector stage comprising a baffle across the housinghaving a flow restricting orifice or opening of reduced cross-sectionand lower spray means is provided for directing scrubbing liquid intothe high velocity gas flow as it passes into and through the opening ina concurrent direction therewith.

With a scrubbing liquor comprising a .wet limestone slurry a reactionis'obtained with the oxides of sulphur in the gas forming calciumsulphite and/or calcium sulphates and this material is'precipitated outof the gas flow and eventually moves to the lower end of the housing ina collection sump below the gas inlet. The concentrated material thusremoved leaves the gas in a clean condition substantially free ofsulphur dioxide and particulates and the gas passes out via the outletat the upper end of the housing.

For a better understanding of the invention reference should be had tothe following detailed description taken in conjunction with thedrawings in which:

FIG. 1 is a vertical cross sectional view of a new and improved wetscrubber system constructed in accordance with the features of thepresent invention;

FIG. 2 is a schematic diagram illustrating the scrubber of FIG. 1 inconjunction with a slurry system for providing scrubbing liquor of wetlimestone slurry for the scrubber; and

FIGS. 3-7 are graphical representations illustrating operatingcharacteristics of the wet scrubber system of the invention whenoperating at various different parameters and variables.

Referring now more particularly to the drawings, in FIGS. 1 and 2 isillustrated a new and improved wet scrubber constructed in accordancewith the features of the present invention and referred to generally bythe reference numeral 10. The wet scrubber is especially well adaptedfor treating industrial gases and the like which contain objectionable.amounts of oxides of sulphur as well as objectionable particulatematerial such as flyash and the like. The scrubber is designed for usein conjunction with a system 12 which furnishes to the scrubber a supplyof scrubbing liquor comprising a wet limestone slurry of crushedlimestone rock and water. Other types of scrubbing liquors may be usedin the scrubber as discussed hereinafter.

The scrubber includes an upstanding housing 14 preferably of rectangularor square cross section and including vertical side walls 16, preferablyof sheet metal having suitable strength and characteristics forresisting corrosion. At the lower end, the housing 14 is provided with asump 15 having inwardly and downwardly sloping bottom walls 18whichconverge at a lower drain outlet 20 for discharging the collectedcontaminated scrubbing liquid back into the slurry system 12 via thesuitable drain fittings such as an elbow 22 and drain line 24. The sump15 in the lower end of the housing 14 is used to collect the scrubbingliquid along with the contaminants absorbed therein obtained from thegas being cleaned in the scrubber housing.

The contaminated gas to be treated is introduced into the lower end ofthe housing 14 via an inlet fitting 26 supplied by. a duct 28; connectedto the source of contaminated gas (FIG. 2). Clean gas is discharged fromthe upper end of the housing 14 from a collecting bonnet 30 having anoutlet 32 connected to a suitable discharge duct 34 (FIG. 2). A fan orblower (not shown) is provided for moving the gas through the scrubberhousing and duct system at the desired flow velocity and pressure. Theblower or fan may be located on the outlet or discharge side of thesystems or on the inlet side of the system as required for a particularinstallation. The gases entering the lower end of the housing throughthe inlet 26 are turned upwardly and flow is a generally verticalpathtoward the discharge bonnet 30 at the upper end of the housing andsome of the heavier and larger contaminant particles therein gravitateout of the gas flow and collect in the sump 15 because of inertialforces as the gases turn to flow in an upward direction.

In accordance with the present invention, the scrubber 10 includes aparticulate removal stage referred to generally by the reference numeral36 and positioned above the gas inlet 26. Scrubbing liquid such as wateris sprayed into the gas flow and is directed concurrently with theupward gas flow in the particulate stage 36 by means of a plurality oflower spray nozzles 38, which nozzles are supplied from amanifold 40.The nozzles are positioned in the housing to spray finely divideddroplets of liquid generally upwardly in a direction concurrent with theupwardly flowing gas. The amount and flow rate of the scrubbing liquidsupplied to the manifold 40 is controlled and adjustable to the desiredlevel by means of a control valve 42.

The particulate removal stage 36 includes a horizon tal wall or bafflestructure 44 extending transversely across the housing between oppositeside walls 16 and dividing the housing into an upper and a lowerchamber. The structure 44 is formed with one or more rectangularopenings therein each of which is preferably positioned above a nozzle38. Within each opening there is provided a modular nozzle structure 46including a rectangular frame and a plurality of elongated grids or rods48 extending between one pair of opposite side members of the frame andarranged in a spaced apart parallel array across the flow passagedefined within the frame. The modular structures 46 are preferablysimilar to that shown and described in the aforementioned U. S. Pat. No.3,488,039. The rods 48 are mounted for free rotation about theirlongitudinal axis and are rotatably supported at opposite ends from theside frame members. The spray of water directed upwardly by the nozzles38 impinges on the surfaces of the rods to wet the same and provides aliquid film for collecting the particulates and other wettablecontaminants in the gas as the gas flows turbulently upwardly throughthe openings or slots defined between the rods 48 and frame members. Theeffective total cross sectional gas flow area defined between rods andframe members is determined by the frame size, rod size and spacing andthe number of modules 46 provided on the baffle structure 44. The gasflow through the slots between rods is turbulent and at a high velocitybecause the flow area is smaller than the overall cross sectional flowarea of the scrubber housing 16. The velocity of the gas flow is greatlyaccelerated as the gas moves through the slots between the pairs ofadjacent rods 48 and turbulent flow causes the particulate matter in thegas to become wetted as the gas contacts the liquid film on the surfaceof the rods and droplets carried in the gas flow. The contaminantmaterial in the gas impinges and collects in the liquid forming liquiddroplets and these liquid droplets move downwardly and collect in thesump 15.

After the gas passes between the rods 48 of the first or lowerparticulate removal, wet scrubbing stage 36 adjacent the lower end ofthe scrubber housing it moves upwardly towards the outlet plenum orbonnet 30 at a substantially reduced average velocity, which velocity isdependent upon the pressure and density of the gas and the whole crosssectional flow area of the housing. Above the particulate separationstage 36, the gas passes through a series of vertically stacked, wetscrubbing stages referred to by the reference numerals 50-1, 2, 3, 4, 5,6, 7, 8 etc. Each of these scrubbing stages includes a plurality ofspaced apart, parallel, elongated rods or tubes 52 arranged in ahorizontal array and extending between opposite side walls 16transversely across the scrubber housing. The vertical spacing distancebetween the successive stages of rods is set up to provide a greatdegree of independence between the scrubbing action accomplished at eachdifferent level or zone. For example, scrubbing liquid cascadingdownwardly from the upper end of the scrubber housing 14 strikes therods 52 in each of the layers in descending order and is deflectedupwardly and outwardly from the rods for a brief interval. During thisbrief interval the liquid droplets are buoyed upwardly by the force ofthe high velocity turbulent gas flow. The scrubbing liquid droplets growlarger as contaminants are collected and precipitates form therein andeventually the droplets gravitate downwardly to the next lower,relatively independent, scrubbing zone or stage whereat similar actiontakes place in the immediate level or vicinity of the array or layer ofparallel rods 52 of the zone.

In describing the visual appearance of the scrubbing action in thescrubber as the scrubbing liquor cascades downwardly to eachsubsequently lower array of rods 52 in the next successive lowerscrubbing stage 50-1, 2, 3, 4, 5, 6, 7, 8, it appears that the scrubbingliquor is somewhat like a waterfall. After reaching the upper scrubbingstages 50-8 and 50-7 etc., the contaminated gas has been substantiallycleansed and has a lower level of contaminant concentration therein.Because of this, it is more difficult to absorb the remainingcontaminants so that in these stages the incoming fresh scrubbing liquoris most effective. As the liquor cascades downwardly from stagecontaminants are absorbed therein until the lower scrubbing stages 50-1and 50-2 are reached. The liquor collects'on the baffle structure ordivider wall 44 and is drained to the sump intermittently through one ormore sock-type drain valves 47. The valves 47 include a normally closedrubber sock at the lower end of a drain tube andwhen the liquid headabove the sock reaches a high enough'level the sock is opened todischarge the liquid and then closes again after the discharge iscompleted.

The gas flows upwardly in a connection generally countercurre'nt to thedownwardly cascading liquid flow moving from the upper scrubbing stage50-1 to the lower stage 50-8 and as the gas reaches each successivehigher scrubbing stage theflow is momentarily acceler ated as it flowsbetween the rods 52 which decrease the effective flow area cross-sectionand thereby increases the flow velocity. The highly turbulent, highvelocity gas flow adjacent the level of the rods in each stage providesan upward buoyant fountain effect which tends to hold and retain thedownward travel of the liquid droplets momentarily in the area or zonejust above and between the rods. This fountain effect at each zonepermits a longer time for intermixing and intensive contact between theliquid and gas at each level. The buoyant fountain effect momentarilycauses the liquid to reverse its downward flow directions in thevicinity or level of. the rods at each stage and the liquid moves upwardly for a brief period in a direction concurrent with the upwardlyflowing gas. Asthe size of the liquid droplets and the density thereofbecomes high enough to overcome the buoyant effect of the high velocitygas the droplets then begin to fall back downwardly. This fountaineffect just described permits the smaller, less denser liquid dropletsto remain in the gas stream for a longer period of time until fullcollection efficiency is obtained before the droplets grow larger andheavy enough to cascade back downwardly to the next lower stage. As thedroplets reach the next lower stage some droplets strike the rods andthe impact breaks the dropto stage, more and more lets up into smallersize droplets which remain in the vicinity until they grow in size andweight enough to overcome the buoyant fountain effect and fall to thenext lower stages. 7

The number of successive vertically spaced scrub-. bing stages or zonesmay be selectively adjusted for a particular application and' the amountof vertical spacing between stages and the open spacing or distancebetween the rods in each layer as well as the diameter of the rods maybe selected to provide a desired particular gas velocity and water ratefor a particular application. The rods are mounted for free rotarymovement about their longitudinal axes and the surfacesof the rods maycontinuously rotate so that the heavier accumulation of precipitatematerial tend to face downwardly toward the upwardly flowing gas. Thus,each stage isselfcleaning and little or no plugging up or clogging overof the spaces between the rods occurs even though materials such ascalcium sulphates and sulphites which tend to precipitate or plate outrather easily are present.

It has been found that the direction of gas flow relative to water flowhas little effect on the efficiency of the collection of particulatesfrom the gas and the direction of water spray in the particulatecollection stage 36 may be either upward or downward. In the S0absorption process in the upper chamber of the housing 16, whether thegas flow is up or down relative to the general downward direction ofliquid flow does have a considerable effect on the efficiency ofabsorption. For an example in a scrubber having a given rodconfiguration and vertical spacing between stages and at a given gasflow rate and water rate, the absorption efficiency for an upwarddirection of gas flow is around 35 percent of SO removed in contrast toa 20 percent S0 removal when the direction of the gas flow is reversedto a downward direction,

The difference in $0 absorption efficiencies between upward and downwardgas flow is believed to be due to the fountain effect wherein thescrubbing liquor striking a layer of rods in a stage momentarily gushesupwardly at this level helped by the influence of the upward gas flowand then cascades back downwardly toward the drainage deck 44 while thewater droplets contact the gas a second time. This arrangement is incontrast to a constant downward flow where the droplets are merelydispersed by the rods and after a brief period of contact with the gas,the liquid reaches the sump. For a given distance of scrubber height anupward gas flow arrangement provides for greater contact time betweenthe liquid and gas due the fountain effect occurring at each rod stagewherein the gas is contacted by the droplets as the droplets momentarilymove upwardly before again falling back down wardly. To provide the sameamount of gas-liquid contact time with both the gas and the liquidflowing in the same direction a much great scrubber height is requiredwith a resulting higher cost. The present scrubber 10 thus provides amost efficient manner of removing the particulates and the S0 inindustrial gases by providing a first lower or initial particulateremoval stage 36 wherein the upward gas flow has relatively littleeffect on particulate removal efficiency followed by a second S0absorbing section comprising a plurality of vertically spaced rod stages50-1, 2, etc. wherein an upward gas flow is extremely efficient in termsof SO absorption and most advantageous in comparison to a downward gasflow arrangement.

Another benefit of the lower rod modules 46 of the particulate stage 36is that the modules act to distribute the gas flow more uniformly overthe entire crosssection of the housing upwardly thereof. The phenomenonof nonuniform gas distribution over the flow crosssection of a vessel isespecially prevalent at relatively low free space gas velocities andefforts to promote a more uniform gas velocity distribution have in theprior art been relegated to devices such as screens which offer asubstantial flow resistance. Screens are generally difficult to use withscrubbing liquids such as limestone slurries because of a tendency toplug-up rapidly with resulting operational difficulties. The rod modules46 on the baffle 44 have acted to more evenly distribute the gasvelocity over the entire cross section of the housing with aconsiderably lower pressure drop than a screen andwith little or noproblem of plugging up.

The rods 52 in each stage 50-1 through 50-8 provide additionalmechanical strength between opposite sidewalls 16 of the scrubberhousing 14 and the rods are readily 'removablefor replacement andcleaning when desired. One or both of the'side walls 16 of the scrubberhousing 14 parallel with the rods 52 is provided with an access openingnormally sealed by a removable access door (not shown) so that a workmanmay have access to the interior of the scrubber housing for removing andreplacement on the rods asrequired. Referring to FIG. 1A, each rod 52 issupported for free rotation at opposite ends by means of a pair ofremovable pins or axles 54 having enlarged heads at their outer ends.The pins are threadedly received ,in openings provided in the walls 16of the housing.

In accordance with the present invention, a scrubbing liquor comprisinga mixture of water and crushed limestone rock in a wet slurry isintermixed with the gas and is introduced into the upper end of thescrubber housing 14' adjacent the level of the upper scrubbing stage0-8. The slurry is directed downwardly toward the rods 52 of the upperscrubber stage through a plurality of ceramic body, spray nozzles 56spaced apart at appropriate intervals above the rods of the upperscrubbing stage 50-8. The spray nozzles 56 are supplied with freshlimestone slurry or a combination of recycled slurry with freshlimestone slurry added as makeup through a piping manifold 58 and a flowcontrol valve 60 is provided to regulate the desired flow rate of freshscrubbing liquid into the system. The liquid-to-gas ratio used in thescrubber may be adjusted as desired by regulating the valve 60. Theslurry system is designed to provide for a wide range of liquid-to-gasratios varying from approximately 5 to 60 gallons of liquid slurry perthousand cubic feet per minute of gas flow. The L/G ratio used isdependent upon the type of gas being treated and the quantity and typeof contaminants carried therein, as well as the amount of contaminantsthat need to. be removed in the process to meet the particularrequirements of the installation as set up by applicable pollutioncontrol standards and codes. The liquidto-gas ratio or L/G ratio is alsoknown as the Water Rate for a scrubber and is readily adjustable byopening or closing the control valve 60. The average gas velocity orflow rate of the gas through the scrubber is measured by a Pitot tube orother suitable instrument and depends upon the gas input to the scrubberhous- 8 ing. The average flow velocity of the gas is measured in termsof inches of water" pressure loss through the scrubber housing frominlet to outlet and may be varied by changing the speed or horsepowerapplied to the fan or blower used for moving the gas through the system.

As previously indicated, the number of scrubbing stages 50-1, 2 etc. maybe changed as required and the vertical spacing between successivestages may also be adjusted as required. The diameter and number of rods52 in each scrubbing stage for a housing of given cross section effectsspacing or area between adjacent rods in each array or layer and thisaccordingly effects the gas velocity and pressure loss.

In FIGS. 3 through 7, test characteristics of the scrubber 10 arepresented. In the graphs of FIGS. 3, 4, 5 and 7 the ordinate representspercent of Sulphur Di oxide removed from the gas and the abscissarepresents -the pressure loss of the gas flowing through the scrubber10. Test gases containing a known quantity of Sulphur Dioxide were usedand the. percent removal figures were calculated by measuring thedifference in Sulphur Dioxide levels in the test gas between samplestaken at the inlet 26 and at the outlet 32 of the scrubber housing.

The graph of FIG. '3 indicates the effect of various different operatingparameters on the absorption efficiency of the scrubber. In general, thedata indicates that more scrubbing stages provide higher absorptionefficiencies but require more horsepower or energy as indicated by thehigher pressure losses. In each case, the rods 52 of each stage werespaced apart to provide an open area for gas flow in the spaces betweenthe rods of a total of 46 percent of the gross transversecross-sectional flow area of the housing. Tests were conducted with fourstages, six stages and eight stages, with the rods in each stageidentical and a vertical spacing between stages of I6 inches. Theaverage gas velocity was held constant at 10 feet per second and foreach scrubber configuration, the L/G ratio was variedas indicated from40 gallons per 1,000 cubic feet per minute to as high as I00. Ingeneral, for a particular number of stages an increase in water rateresulted in an .increase in absorption efficiency with a higher pressureloss resulting. i

The graph of FIG. 4 illustrates more definitively the effect of varyingthe water rate and the gas velocity for a given number of scrubbingstages. In these tests, eight scrubbing stages were provided and eachstage includes 3 1 inch round pipes or rods spaced apart to provide a 41percent open area. The stages were spaced at I6 inch intervals. Thegraph shows that higher gas velocity results in a substantiallyunchanged absorption efficiency if the L/G ratio is maintained constantand at a constant gas velocity an increase in water rate produces asubstantial increase in absorption efficiency. This would indicate thatwater rate is far more important in achieving improved absorption thangas velocity.

, FIG. 5 illustrates the effect of rod configuration on absorptionefficiency. For a given number of stages it appears that decreasing theopen area and hence increasing the surface area of the rods whilemaintaining a relatively constant gas velocity improves the absorptionefficiency. In this area the curves are rather steep indicating thatmore rod surface area available does increase absorption efficiency at arelatively lower cost in terms of pressure loss or power requirementsthan occurs when larger diameter rods are provided. This .nozzles 68which are supplied graph indicates that it might be better to provide alarger number of small diameter rods than to provide a smaller number oflarger diameter rods by comparison with the results of FIG. 4 wherein.larger diameter rods are used. The graph of FIG. 6 brings out thisconclusion more clearly when comparing the results of the A inchdiameter rod stages A and the A inch diameter rod stage B operated atthe same water rates. In the graph of FIG. 6, the abscissa is in termsof horsepower per thousand cubic foot a minute gas flow and the formulafor horsepower calculation is indicated. The curve designated C is drawnfor a scrubber wherein a wire mesh grid of 46 percent open area is usedfor each stage rather than rods in'one direction only. The curvedesignated E is drawn for a scrubber using two layers or. stages ofceramic packing materials in spaced apart bed, each bed being 12 inchesdeep like that shown in U.S. Pat. No. 2,253,261, and the curve F" isdrawn for a scrubber using a marble bed as a filter element like thescrubber shown in US. Pat. No.

In FIG. 7, the curve shows the difference in operating results between 8stages of 3/16 inch rods with a 40 percent open area and 8 stages ofscreen mesh with a 46 percent open area. The screen mesh islessefficient probably because of cross-strands and increased frictionand the screen requires additional support structure for carrying thesame while the rods are selfsupporting and lend structural strength tothe scrubber housing 14. in both the rod stages and the screen stages,the liquor is supplied from outlets comprising straight pipe nipplesrather than ceramic spray nozzles 56 and this reduces the efficiencysomewhat because of poorer mixing. In the curve labeled with waterdeflectors plates are mounted some 3 to inches below the outlet end ofthe nipples to deflect the discharge horizOntally outwardly. Thisappeared to improve efficiency at the lower end. After the gases exitthe uppermost scrubbing stage 50-8 in the scrubber housing 14 they flowupwardly into a final demisting section generally indicated by thereference numeral 62. The demisting section is separated and supportedfrom the lower portion of the housing 14 by means of an upper,horizontal baffle 64 between the outlet bonnet 30 and the upper spraynozzles 56. The baffle 64 is provided with one or more elongated slotsor openings therein to accommodate a row of vertically upstanding,demisting vanes 66, having a generally Z-shaped horizontal crosssection. Each vane includes three planar sections arranged at an acuteangle relative to the next-adjacent section to rapidly change the gasflow direction. A flow of fresh makeup water is sprayed onto the lowersegments of the demisting vanes via a plurality of spray from a manifoldsupply line 70 having a regulating control valve 72 therein forcontrolling the flow rate of makeup water to the system. Any liquidcarried upward by the buoyant force of the gas flow is in the form offine droplets or mist and the liquid mist impacts against and collectson the surfaces of the Z-shaped vanes 66. This liquid flows downwardlyon the vane surfaces washing off the vanes and eventually movesdownwardly into the main liquid flow in the scrubber housing. Referenceshould be had to US. Pat. No. 3,289,398 for a more complete descriptionand operation of the demisting vane system 62.

It has been found that a controlled spray of fresh liquid from thenozzles 68 on the lower portions of the vane 66 keeps the vanes cleanand free of material and the makeup water continually washes down thevanes without substantially increasing the moisture content of the gasleaving the scrubber bonnet 30. Fresh makeup 1 water may also be addedin the particulate stage 36 via the nozzles 38.

ln accordance with the present invention and referring specifically toFIG. 2 the limestone slurry supply system 12 is adapted to provide aworking supply of crushed limestone rock mixed with water in slurry formhaving a desired percentage of concentration. Slurry is supplied via thecontrol valves 42 and 60 which in turn regulates the flow of the mixtureinto the respective manifolds 40 and 58. Liquid is supplied to thevalve60 from a pressure line 74 connected to the outlet side of a pressurepump 76 for pumping the slurry into the scrubber housing. The inlet sideof the pump 76 is supplied with a slurry having a 15 percentconcentration of solids consisting partly of crushed limestone andpartly of fly ash in water via an inlet line 76 leading from a slurryholding tank 80. In the holding tank the crushed limestone rock in theslurry mixture is prevented from settling to the bottom of the tank-bymeans of an agitator 82 driven by a motor 84 and shaft 86. New slurry issupplied to the holding tank in the form of a mixture having a higherconcentration of limestone therein via a supply line 88 connected to theoutlet of a transfer pump 90. The inlet side of the transfer pump issupplied with a concentrated slurry mixture via an inlet line 92connected to a slurry holding tank 94 which contains a quantity ofslurry having a concentration of approximately 66 percent limestone rockin water. The tank 94 is also provided with a rotating agitator 96driven by a shaft 98 and motor 100 to prevent the limestone fromsettling to the bottom of the tank.

In order to reduce the concentration of the 66 percent slurry deliveredto the 15 percent slurry holding tank 80 via the line 83, the tank 80 isalso supplied with reclaimed liquid from a settlement system (notshown). This liquid has a relatively low concentration of limestone andflows into the tank 80 via a supply line 102. The liquid suppliedthrough the line 102 generally comprises a clear liquid which isobtained from the discharge material of the scrubber 10, after thematerial has been treated in a settlement system to remove the calciumsulphate, calcium sulphite and other contaminants therefrom which werecollected in the scrubbing process in the scrubber 10. if desired,additional fresh makeup water can be supplied directly to the 15 percentholding tank 80 through a fresh water line 104 having a control valvel06therein. The fresh water makeup supplied via the line 104 and the clearwater returned to the line 102 are regulated and intermixed with theincoming high concentration slurry supplied via the line 88 from thetank 94 so that the concentration of limestone in the slurry held in thetank 80 is maintained at the desired concentration.

Reference should be had to the United States copending patentapplication Ser. No. 295,006 filed Oct. 4, 1972 and assigned to the sameassignee as the present invention in which a slurry system like thesystem 12 is described in more detail. This copending application isincorporated herein by reference and it is sufficient to say herein thata portion of the material drained from the sump 26 from the scrubberhousing 12 may be siphoned directly into the slurry tank 80 1 1 whileanotherportion thereof is treated to remove the heavy sludge materialtherefrom in a settlement system (not shown). As described in theforegoing copending United States patent application limestone which isavailable in quantity at relatively low cost is supplied from railroadcars 108 or other conveyances to an un loading station 110 havingelevating means 112 for delivering the material to a yard storage area114. Preferably the material is broken or crushed to provide an averageone inch particle size. Limestone from the storage area 114 is deliveredin an elevator 116 into the feeder 118 of a ball mill crusher 120. Wateris added to the ball mill through a line 122 and a control valve 124 inthe line is adjustable to control the wetness of the limestone as it iscrushed in the ball mill. Discharge from the ball mill 120 is directedinto a holding tank v 126 and additional water may be added via 'asupply line 128 with a control valve 130 therein. The holding tank 126is continuously agitated by an agitator 132 and material is pumpedtherefrom via a line 134, pump 136 and pump outlet line 138 into ascreen classifier 140 wherein oversized material is directed via a chute142 back to the inlet side of the ball mill. The on-size material flowsvia a line 144 to the inlet side of a pump 146 which feeds the 66percent slurry holding tank 94 via a line 148. 1

The foregoing slurry system 12 and scrubber 10 provide an exceptionallyeffective and economically soundsystem for removing sulphur dioxide fromindustrial gases and the like. The rod stages or zones employed by themulti-stage scrubber lfl are far superior to a packed tower system whichis unuseable because of its tendency to rapidly plug up withprecipitated out materials. The rods provide mechanical strength for thescrubber housing 14 and can be arranged to alternately run intransversedirections to provide additional mechanical strength.

In accordance with the invention a scrubbing liquid comprising a mixtureof slaked line (Ca(OH) and water is also useful in absorbing SO -fromthe gas being treated in thesc'rubbing zones 50-1, 2. etc. of thescrubber 10. The slaked lime mixture is extremely reactive and efficientin absorbing S however, this advantage may be offset somewhat by thehigher cost of slaked lime in comparison to crushed limestone. Thishigher cost may be offset, however, by the fact that because of thehigher reactiveness of a slaked lime slurry with S0 in the gas, lesscontact time is required between the liquid and gas and hence a scrubberwith fewer scrubbing zones 50-1, 2 etc. can be utilized at a consequentlower cost.

Another type of scrubbing liquid which can be used in the scrubbercomprises a mixture of Magnesium Hydroxide (Mg,(Ol-l) and scrubbingliquids comprising mixtures of water and soluble alkalis such as S0-dium Hydroxide (NaOH) and Ammonium Hydroxide (NH,OH) are useful. Thesetypes of scrubbing liquid dont seem to have the problem of plugging upin the scrubber 10 of the present invention as they often do when usedin conventional packed tower type scrubbers. The reaction productsformed when the S0 is absorbed in these scrubbing liquids is somewhatsoluble, however, suitable regeneration techniques are available'toremove the reaction products from the scrubbing liquid so that theliquid may be reused. In many cases the reaction products so removed areeconomically valuable as fertilizers and process chemicals.

I Although the present invention has been described with reference to asingle illustrative embodiment thereof, it should be understood thatnumerous-other modifications and embodiments can be devised by thoseskilled in the art that will fall within the spirit and scope of theprinciples of this invention.

What is claimed as new and desired to be secured by letters Patent ofthe United States is:

l. A wet scrubber forabsorbing oxides of sulphur from contaminated gascomprising: an upright housing having an inlet for contaminated gasadjacent the lower end and an outlet for clean gas adjacent the upperend; a plurality of vertically stacked, intense wet scrubbing zones insaid housing between said inlet and outlet, each zone comprising atransversely extending array of elongated, parallel, spaced apart,rod-like elements substantially reducing the total flow cross sectionfor said gas flowing between adjacent elements below that of the flowcross section of said housing above and below said array; upper spraymeans adjacent an upper one of said scrubbing zones for distributingfinely divided droplets of scrubbing liquor into said gas adjacent saidelements to cascade downwardly countercurrent to said gas flow intosuccessive lower scrubbing zonesin said housing; and a highvelocityparticulate collector stage in said housing between said inletand a lower one of said wet scrubbing zones comprising baffle meansacross said housing having a flow restricting opening therein and lowerspray means for directing scrubbing liquid into the high velocityflowing through said opening in a concurrent direction therewith. i

2. The wet scrubber of claim 1 wherein said particulate collector stagecomprises a transversely extending baffle between said inlet and saidlower one of said scrubbing zones, said baffle having at least oneopening therein having a cross-section substantially less than that ofsaid housing for causing accelerated upward gas flow therethrough and aplurality of elongated rod-like elements extending across said openinghaving impingement surfaces for receiving spray from said lower spraymeans for wetting and collecting particulant contaminants from said gas.

3. The scrubber of claim 1 including means for supporting said rod-likeelements at opposite ends from walls of said housing permitting freerotation of said elements around their longitudinal axes.

'4. The scrubber of claim 1 including a row of vanes between an upperone of said scrubbing zones and said outlet, each vane including atleast a pair of vertically extending intersecting impingement surfacesin parallel spaced apart relation with surfaces of an adjacent vane insaid row for collecting liquid from said gas flowing between saidsurfaces of said adjacent vanes.

5. The scrubber of claim 4 including means for washing down said vanesin said row with liquid to prevent accumulation of material on saidsurfaces thereof.

6. The scrubber of claim 1 in combination with a system for producing aslurry of crushed limestone and water for delivery to said upper spraymeans.

7. The wet scrubber of claim 2 including a collection sump adjacent thelower end of said housing below said inlet for collecting precipitousmaterial formed by the mixing of said gas and said scrubbing liquid, anddrain means for removing said collected materials from said housing. =is

2. The wet scrubber of claim 1 wherein said particulate collector stagecomprises a transversely extending baffle between said inlet and saidlower one of said scrubbing zones, said baffle having at least oneopening therein having a cross-section substantially less than that ofsaid housing for causing accelerated upward gas flow therethrough and aplurality of elongated rod-like elements extending across said openinghaving impingement surfaces for receiving spray from said lower spraymeans for wetting and collecting particulant contaminants from said gas.3. The scrubber of claim 1 including means for supporting said rod-likeelements at opposite ends from walls of said housing permitting freerotation of said elements around their longitudinal axes.
 4. Thescrubber of claim 1 including a row of vanes between an upper one ofsaid scrubbing zones and said outlet, each vane including at least apair of vertically extending intersecting impingement surfaces inparallel spaced apart relation with surfaces of an adjacent vane in saidrow for collecting liquid from said gas flowing between said surfaces ofsaid adjacent vanes.
 5. The scrubber of claim 4 including means forwashing down said vanes in said row with liquid to prevent accumulationof material on said surfaces thereof.
 6. The scrubber of claim 1 incombination with a system for producing a slurry of crushed limestoneand water for delivery to said upper spray means.
 7. The wet scrubber ofclaim 2 including a collection sump adjacent the lower end of saidhousing below said inlet for collecting precipitous material formed bythe mixing of said gas and said scrubbing liquid, and drain means forremoving said collected materials from said housing.